A throttle body is mounted to a fixed member, such as an intake manifold of an internal combustion engine. The fixed member has a connecting portion such as a connecting face that may be varied in dimension due to manufacturing tolerance or aging. A conventional throttle valve has a cylindrical portion forming an intake air passage. The cylindrical portion may deform due to strain arising in the fixed member.
Accordingly, a shut off position of a conventional throttle valve is set at a position in which the throttle valve slightly opens relative to a position, in which the throttle valve makes contact with the cylindrical portion, considering a variation in dimension. In this structure, leakage of intake air increases in the shut off position. Besides, when the shut off position is set at a position, in which the throttle valve is further closed, seizure may occur between the throttle valve and the inner wall of the cylindrical portion. By contrast, when leakage of intake air is large in a condition, in which the throttle valve is in the shut off position, the engine may not be capable of adapting to a low idling standard of the engine for reduction in fuel consumption.
In addition, a throttle body of an intake air control device is conventionally formed of a metallic material such as aluminum. However, in recent years, a throttle body is formed of resin for reduction in cost and weight.
The resinous throttle body is apt to deform due to its material property. Accordingly, strain of the fixed member may be transmitted to a cylindrical member, which receives a throttle valve, via a flange in the throttle body, and the cylindrical member may be deformed. Here, the flange of the throttle body is a sealing collar portion that includes multiple fixed portions to be connected to the fixed member. When the cylindrical member deforms, leakage of intake air may increase, and seizure may occur between the throttle valve and the inner surface of the cylindrical portion.
According to conventional structures disclosed in JP-A-10-280981, JP-A-11-013562, and JP-A-11-062739, as shown in FIG. 13, a thin wall portion J5, which is a low rigidity portion for absorbing strain, is provided between a fixed portion J2 and a cylindrical portion J4. The fixed portion J2 is fixed to the fixed member using a securing member such as a bolt and a screw J3. When strain is transmitted from the connecting portion of the securing member J3 to the fixed portion J2, the thin wall portion J5 is deformed, so that the strain is restricted from being transmitted to the cylindrical portion J4.
However, the thin wall portion J5 can be formed to be only a small portion between the fixed portion J2 and the cylindrical portion J4. Accordingly, the thin wall portion J5 may not sufficiently absorb strain. Therefore, strain transmitted to the fixed portion J2 may be further transmitted to the cylindrical portion J4 via the thin wall portion J5. As a result, the cylindrical portion J4 may be deformed.